It really all depends on how much capital you can raise, knowledge and how much risk you can take.
It helps to have a rich daddy.
My current favourite is to start an onland fish farm. A study has shown that fish caught at sea etc. on average 1 in 3 has ingested plastic. Not only will most of the fish in the oceans die off but the public will no longer want to eat fish.
However large tanks of fresh flowing water can raise clean and healthy fish. I don't know much about the detail but with a rising population this could be a real money spinner.